In the fabrication of semiconductor integrated circuits, metal conductor lines are used to interconnect the multiple components in device circuits on a semiconductor wafer. A general process used in the deposition of metal conductor line patterns on semiconductor wafers includes deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal conductor line pattern, using standard lithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby leaving the metal layer in the form of the masked conductor line pattern; and removing the mask layer typically using reactive plasma and chlorine gas, thereby exposing the top surface of the metal conductor lines. Typically, multiple alternating layers of electrically conductive and insulative materials are sequentially deposited on the wafer substrate, and conductive layers at different levels on the wafer may be electrically connected to each other by etching vias, or openings, in the insulative layers and filling the vias using aluminum, tungsten or other metal to establish electrical connection between the conductive layers.
Electrodeposition or electroplating of metals on wafer substrates has recently been identified as a promising technique for depositing conductive layers on the substrates in the manufacture of integrated circuits and flat panel displays. Such electrodeposition processes have been used to achieve deposition of the copper or other metal layer with a smooth, level or uniform top surface. Consequently, much effort is currently focused on the design of electroplating hardware and chemistry to achieve high-quality films or layers which are uniform across the entire surface of the substrates and which are capable of filling or conforming to very small device features. Copper has been found to be particularly advantageous as an electroplating metal.
Electroplated copper provides several advantages over electroplated aluminum when used in integrated circuit (IC) applications. Copper is less electrically resistive than aluminum and is thus capable of higher frequencies of operation. Furthermore, copper is more resistant to electromigration (EM) than is aluminum. This provides an overall enhancement in the reliability of semiconductor devices because circuits which have higher current densities and/or lower resistance to EM have a tendency to develop voids or open circuits in their metallic interconnects. These voids or open circuits may cause device failure or burn-in.
A typical standard or conventional electroplating system includes a standard electroplating cell having an adjustable current source, a bath container which holds an electrolyte electroplating bath solution (typically acid copper sulfate solution), and a copper anode and a cathode immersed in the electrolyte solution. The cathode includes a semiconductor wafer that is to be electroplated with metal. A contact ring typically mounts the wafer to the cathode. Both the anode and the cathode are connected to the current source typically by means of suitable wiring.
In operation of the electroplating system, the current source applies a selected voltage potential between the anode and the cathode. This potential creates a electrical field around the anode and the cathode, which electrical field affects the distribution of the copper ions in the bath. In a typical copper electroplating application, a voltage potential of about 0.1˜20 volts may be applied for about 2 minutes, and a current of about 0.1˜20 amps flows between the anode and the cathode and wafer. Consequently, copper is oxidized at the anode as electrons from the copper anode reduce the ionic copper in the copper sulfate solution bath to form a copper electroplate on the wafer, at the interface between the wafer and the copper sulfate bath.
The copper oxidation reaction which takes place at the anode is illustrated by the following reaction equation:Cu→Cu+++2e−
The oxidized copper cation reaction product forms ionic copper sulfate in solution with the sulfate anion in the bath:Cu+++SO4−−→Cu++SO4−−
At the wafer, the electrons harvested from the anode flowed through the wiring reduce copper cations in solution in the copper sulfate bath to electroplate the reduced copper onto the wafer:Cu+++2e−→Cu
When a copper layer is deposited on the wafer, such as by electrochemical plating, the copper layer must be deposited on a metal seed layer such as copper which is deposited on the wafer prior to the copper ECP process. Seed layers may be applied to the substrate using any of a variety of methods, such as by physical vapor deposition (PVD) and chemical vapor deposition (PVD). Typically, metal seed layers are thin (about 50-1,500 angstroms thick) in comparison to conductive metal layers deposited on a semiconductor wafer substrate.
Conventional electrochemical plating techniques typically use sulfuric acid (H2SO4) as the electroplating chemical in the electroplating bath solution. The solution may further include additives such as chloride ion and levelers, as well as accelerators and suppressors which increase and decrease, respectively, the rate of the electroplating process. The rate of deposition of copper on the substrate, and the quality and resulting electrical and mechanical properties of the metallization, are largely dependent on the concentration of these organic additives in the electroplating bath solution.
However, one of the drawbacks of using sulfuric acid as the electroplating chemical is that the sulfuric acid has a tendency to damage the copper seed layer and cause poor gap-filling. One method which is used to ameliorate this phenomenon involves applying a bias current to the wafer prior to immersing the wafer in the electroplating bath solution. Upon immersion of the wafer, the circuit between the current source and the cathode and anode is closed, and the electroplating process begins. However, this pre-applied bias activates the electroplating tool alarm if the circuit remains open for too long. Furthermore, a sudden current spike results upon entry of the wafer into the electroplating bath solution, causing local defects and non-uniform plating.
Accordingly, an ECP apparatus and method are needed to facilitate uniform current distribution on a wafer during loading of the wafer in the ECP apparatus.